The present invention relates to a glass compression molding machine and a system employing same.
Previously, when an optical glass lens, for example, was molded, glass which had been melted and solidified was rough cut, subjected to grinding and so forth to form a glass preform of predetermined shape. Then, the preform was placed into a molding die assembly having precise molding surfaces to compress the preform with heating, followed by gradually cooling the preform. The molding would then be ejected from the molding die assembly.
FIG. 1 illustrates the state of a molding die assembly of a conventional glass compression molding machine before compression is performed. FIG. 2 illustrates the state of the molding die assembly of the conventional glass compression molding machine after compression has been performed.
Referring to the drawings, reference numeral 1 represents an upper core, 2 represents a lower core, and 3 represents a cylinder disposed between the upper core 1 and the lower core 2 to guide the vertical movement of the upper core 1. The upper core 1 includes a flange portion 1a for contact with an upper heating plate (omitted from illustration), and a column portion 1b formed integrally with the flange portion 1a. A lower surface 1c of the column portion 1b has a shape that corresponds to a molding 8.
The lower core 2 includes a flange portion 2a for contact with a lower heating plate (omitted from illustration) and a column portion 2b formed integrally with the flange portion 2a. An upper surface 2c of the column portion 2b has a shape that corresponds to the molding 8 so that a cavity 4 is formed in cooperation with lower surface 1c of the column portion 1b.
The cylinder 3 is shaped to mate with the column portion 2b of the lower core 2, the lower surface of the cylinder 3 coming into contact with the flange portion 2a of the lower core 2. The upper core 1 forms an upper molding die 6, while the cylinder 3 and the lower core 2 form a lower molding die 7.
With the upper molding die 6 separated from the lower molding die 7, a robot (omitted from illustration) places a preform 5 on the foregoing lower molding die 7. Then, an upper platen (omitted from illustration) is moved downwards closing the die cavity and clamping upper and lower molding dies 6 and 7 together. The preform 5 is then heated and compressed so that the molding 8 is obtained as shown in FIG. 2. The molding 8 is cooled, and then the die assembly is opened by moving the upper platen upward. The molding 8 is removed by a robot or the like.
Therefore, each of the upper platen and lower platen (omitted from illustration) is provided with a preform-injecting/molding-ejecting station, a clamping/heating/compressing station and a gradual-cooling station. Furthermore, independent clamping devices (omitted from illustration) for heating, compressing and gradually cooling are provided.
In compression, a compressive force is applied to the preform 5 in the cavity 4. The relative dimensions of the upper molding die 6 and the lower molding die 7 are set to provide a slight clearance d upon completion of the compression.
The upper molding die 6 and the lower molding die 7 of the foregoing type, generally made of hard metal or ceramic, are used to mold glass at high temperature and high pressure, resulting in adhesion of molten glass to the surfaces of the upper core 1 and the lower core 2. Accordingly, the upper core 1 and the lower core 2 are covered with thin films to prevent the adhesion of the molten glass to their surfaces.
However, to the extent that the upper platen and the lower platen of the foregoing glass compression molding machine are not parallel, the upper molding die 6 and the lower molding die 7 will not be properly aligned. As a result, the surface accuracy and the shape accuracy of the molding 8 are reduced and, in particular, if the molding 8 is a glass lens, the result is distortion of the optical axis of the glass lens.
FIG. 3 is a sectional view explaining the degree of parallelism between the upper platen and the lower platen of the conventional glass compression machine. FIG. 4 illustrates a conventional glass lens product.
In FIG. 3, reference numeral 1 represents an upper core, 2 represents a lower core, 3 represents a cylinder, 6 represents an upper molding die and 7 represents a lower molding die. Reference numeral 21 represents an upper heating plate to be brought into contact with the upper core 1, and 45 represents a lower heating plate to be brought into contact with the lower core 2. The upper heating plate 21 is fastened to the upper platen 9, while the lower heating plate 45 is fastened to the lower platen 10.
If the degree of parallelism between the upper platen 9 and the lower platen 10 of the foregoing molding die assembly is poor, clamping and compressing cause a difference to occur between, for example, front distance H.sub.1 and back distance H.sub.2 between the upper heating plate 21 and the lower heating plate 45.
Referring to FIG. 4, reference numeral 8 represents a glass lens molding having thickness T. Reference numeral 8a represents a first optical surface and 8b represents a second optical surface respectively having radii of curvature R.sub.1 and R.sub.2. If the degree of parallelism between the upper platen 9 (see FIG. 3) and the lower platen 10 is poor, distortion e of the optical axis takes place, causing the thickness of the peripheral ends of the molding 8 to have different values t.sub.1 and t.sub.2. Therefore, sloping of the optical surfaces 8a and 8b expressed by .DELTA.t (=t.sub.2 -t.sub.1) is seen.
Where a plurality of upper molding dies 6 and lower molding dies 7 are moved between the foregoing stations, the degree of parallelism between the upper molding die 6 and the lower molding die 7 may also be poor and, as a result, the surface accuracy and the shape accuracy of the molding 8 become poor.
FIG. 5 is a cross-sectional view illustrating the degree of parallelism between the upper platen and the lower platen of a conventional glass compression molding machine using plural die assemblies. Referring to FIG. 5, reference numeral 6 represents an upper molding die, 7 represents a lower molding die, 8 represents a molding, 9 represents an upper platen, 10 represents a lower platen, 21 represents an upper heating plate and 45 represents a lower heating plate. In this case, a plurality of upper molding dies 6 are brought into contact with the upper heating plate 21 and a plurality of lower molding dies 7 are brought into contact with the lower heating plate 45 in the clamping/heating/compression station.
If the preform 5 (see FIG. 1) has not been previously machined prior to placement on the lower molding die 7, the volume of each preform 5 will vary by a degree of several percent. Differences in volume as between different preforms leads to a reduction in the degree of parallelism between the upper platen 9 and the lower platen 10. In other words, a difference between the front distance H.sub.1 and the back distance H.sub.2, between the upper heating plate 21 and the lower heating plate 45, is seen. As a result, one of the molding die assemblies is compressed excessively, while the other is compressed insufficiently. In this case, thickness T.sub.1 of the front molding 8 and thickness T.sub.2 of the rear molding 8 will be different from each other, thus reducing surface accuracy and shape accuracy.
In the foregoing conventional glass compression molding machine, the upper core 1 and the lower core 2 are covered with thin films to prevent the adhesion of molten glass to their surfaces. As a result, repeated molding operations will repeatedly subject the foregoing thin films to a temperature which is higher than 300.degree. C., causing the thin films to be oxidized. Hence, the durability of the molding die assembly deteriorates.
Therefore, the temperature of the upper molding die 6 and that of the lower molding die 7 cannot be detected accurately, and, accordingly, the control of the temperature cannot be performed accurately.